1. Field of the Invention
This invention relates in general to apparatus for guiding a tape through one, or another, tape path in a tape recorder; more particularly, the invention is concerned with guides which are mounted in a coaxial-reel tape cassette, for guiding magnetic tape selectively through different tape paths, including one path which is so disposed in the tape recorder that a television signal train may be helically recorded on the tape. (As used herein, the term "recorder" shall be taken to mean apparatus which either or both records or plays back video signal information.)
2. Description Relative to the Prior Art
While it is not so restricted, the invention acquires a special significance when it is used to guide magnetic tape contained in a coaxial-reel cassette toward and away from a helical recording drum in a video tape recorder. Coaxial-reel cassettes are particularly well adapted for use with video recorders, which generally tend to be bulky, since cassette takeup and supply reels are rotated on the same axis. As is usual, coaxial-reel cassettes will have a pair of inclined or tapered guide posts which are so oriented within the cassette as to take up the change in tape level between the reels when the tape, fully contained in the cassette, passes directly from one reel to the other.
For helical recording, the tape is initially pulled from the cassette and wrapped around the recording drum at a helix angle. In addition, the helical recording format requires precise positioning of the span of tape which passes around the drum; positioning errors may cause, for example, mistracking during playback. Tracking problems are diminished, however, if the tape follows a precise path into, around, and out of the drum assembly. In one video recorder configuration, the paths into and out of the drum are horizontal and substantially in the same plane as the supply and takeup reels, respectively. The last post before the tape touches the drum and the first post after the tape leaves the drum are designed to change the tape's horizontal level a few degrees so as to dispose the tape properly to form a helix around the drum.
These factors suggest the use of the coaxial-reel cassette with the helical drum assembly; however, this combination is hampered significantly because the tape presented to the helical drum is last touched within the cassette by the inclined or tapered guide posts. What this means is that the last guide surfaces within the cassette will tend to force the tape into an inclined path which is not suitable for presentment to the drum assembly and its associated guides. On the other hand, the tape still needs to be positively guided to the vicinity of the drum at the correct height for proper helical scanning. In an attempt to meet this problem, a multiplicity of guides are commonly provided on the recorder to positively guide the tape intermediate the cassette and the drum. The guides gradually compensate for the abrupt level change between the coaxial reels and finally present the tape at a suitable angle to the drum assembly. While perhaps expedient for a large machine, the size of such a guiding assembly plainly puts restraint on any effort to reduce bulkiness of a typical video tape recorder.
Since tape stability becomes such a formidable problem, the tape should be withdrawn from the cassette in such a manner as to keep the tape essentially free from being stressed differentially across its width as it encounters the cassette tape guides. The problem, therefore, becomes one of guiding the tape in an essentially distortion-free manner through either of two paths: a first path contained within the cassette, which imposes minimal stresses during high speed wind or rewind; a second path extending outside of the cassette which orients the tape to the drum assembly without introducing any stress differentials which would adversely affect the stability of the helix formed around the drum. The first path must additionally take up the level change between the two coaxial reels while the drum assembly and its associated guides account for the level change in the second path.
Approaches taken heretofore to guide tape between coaxial reels have provided tilted conical guide members for guiding a magnetic tape from a reel in one coaxial plane to a reel in another coaxial plane without stretching or distorting the tape, e.g., U.S. Pat. No. 3,042,331. Other arrangements have used cylindrical guides canted with respect to the axis of the reels, e.g., U.S. Pat. No. 2,706,638. These structures are intended for handling the tape between the reels, but they are commonly unsuited for positively guiding the tape in a distortion-free manner if the tape should be drawn away from the reels, as required by helical recording.
Various configurations have been proposed to produce both effects. These include guiding assemblies in the corner of a cassette composed of several posts -- both upright and canted, e.g., U.S. Pat. No. 3,800,314. The tape winds its way from one post to another depending on whether the tape is to remain in the cassette or to be withdrawn from the cassette. However, in passing between this maze of posts, new stress differentials are likely to be impressed on the tape. Other suggestions have included a spring-loaded cassette post which is forced out of the tape path by a fixed post on the recorder deck when the cassette is emplaced on the recorder, e.g., U.S. Pat. No. 3,678,213. In such a manner, two separate guide posts are provided for two different paths. However, high speed winding with the tape contained in the cassette is hindered by the presence of the incorrect guide surface. Other proposals to provide separate guide surfaces for either path tend to occupy to much space within the cassette. Two cylindrical guides, one canted with respect to the coaxial reels, have been spaced apart in a corner of a cassette, e.g., U.S. Pat. No. 3,672,603. When the tape is withdrawn from the cassette, it rides away from the canted guide and is directed only by the upright guide. In addition to size considerations, such an arrangement does not lead to a contiguous guide structure, since making one guide contiguous to the other will leave irregular surfaces that may severely damage the tape.
A copending U.S. patent application Ser. No. 606,995, in the name of Thomas G. Kirn, filed concurrently with and assigned to the same assignee as the present application, provides a rotatable guide, for use within a coaxial-reel cassette, having right circular conical and cylindrical surfaces on opposite sides thereof. By providing such a guide, with appropriate selective orientation, a tape may be translated directly between the reels by means of the conical surface, or guidedly directed to and from a recording drum about which it helically wraps, by means of the cylindrical surface. Although this approach is an effort to meet the size question, the upright conical surface could contribute undesirable stress differentials and distorts the tape as it passes thereabout from one coaxial level to the other.